1. Field of the Invention
This invention relates generally to magnetic head servo control systems and, more particularly, to disk drive position control systems that determine the location of a head relative to disk tracks.
2. Description of the Related Art
In a conventional computer data storage system having a rotating storage medium, such as a magnetic or magneto-optical disk system, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk, for example, can comprise a disk substrate having a surface on which a magnetic material is deposited. The digital data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from and recorded onto the disk surface in close proximity to the disk. That is, a read/write head can produce and detect variations in magnetic orientation of the magnetic material as the disk rotates relative to the head.
Conventionally, the read/write head is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head from data track to data track and, once over a selected track, to maintain the head in a path over the centerline of the selected track. Maintaining the head centered over a track facilitates accurate reading and recording of data in the track. Positioning read/write heads is one of the most critical aspects of recording and retrieving data in disk storage systems. With the very high track density of current disk drives, even the smallest head positioning error can potentially cause a loss of data that a disk drive customer wants to record or read. Accordingly, a great deal of effort is devoted to servo systems.
A servo control system generally maintains a read/write head in a position centered over a track by reading servo information recorded onto the disk surface. The servo information comprises a position-encoded servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in disk servo tracks. The flux transitions are recorded as periodic servo pattern bursts formed as parallel radial stripes in the servo tracks. When the read/write head passes over the servo pattern flux transitions, the head generates an analog signal whose repeating cyclic variations can be demodulated and decoded to indicate the position of the head over the disk. The demodulated servo signal is referred to as a position error sensing (PES) signal.
There are a variety of methods for providing servo track information to a disk servo control system. In a method referred to as the dedicated servo method, the entire surface of one side of a disk is pre-recorded with servo track information. A servo head is positioned over the dedicated servo disk surface in a fixed relationship relative to data read/write heads positioned over one or more other data disk surfaces. The position of the servo head relative to the dedicated disk surface is used to indicate the position of the multiple data read/write heads relative to their respective disk surfaces. The dedicated servo method is most often used with multiple disk systems in which a servo head of a single dedicated servo disk surface controls movement of corresponding data read/write heads of a multiple platter disk drive.
Another method of providing servo track information is known as the sector servo method. In the sector servo method, each disk surface includes servo track information and binary data recorded in concentric or spiral tracks. The tracks on a sector servo disk surface are partitioned by radial sectors having a short servo track information area followed by a data area. The servo track information area typically includes a sector marker, track identification data, and a servo burst pattern. The sector marker indicates to the data read/write head that servo information immediately follows in the track. The servo read head is typically the same head used for reading data.
In both the dedicated servo and sector servo types of systems, the PES signal is used to generate a corrective input signal that is applied to the read/write head positioning servo. The remaining description assumes the sector servo system, but the manner in which the servo control system could be applied to a dedicated servo system will be readily apparent to those skilled in the art.
FIG. 1 is a representation of servo track information pre-recorded into a track 20 of a conventional disk 22 for an exemplary servo sector and data field. An initial field in the track comprises a synchronization field 24, such as for automatic gain control (AGC) or similar signal detecting purposes. The next field in the track is a track identification field 26, typically comprising a digitally encoded gray code. Next is a PES pattern field 28, generally containing a servo pattern burst, as described above. The next field in the track is a customer data synchronization field 30, for permitting read circuitry to adjust to the data amplitude and frequency, which may differ from those of the servo information. The data synchronization field 30 is followed by a customer data field 32.
The track identification field 26 permits an unambiguous numerical identification for data tracks. It provides a coarse track indication for a single track, and the servo pattern provides more precise positioning information within a single track. Because track identifier information identifies a single track, it must be capable of being read from track to track as the servo head is moved across the disk in a track seek operation. Therefore, the magnetic transitions making up the track identification are typically radially aligned with each other. The range of a track identifier (one track) is smaller than the distance over which the servo pattern repeats, so that there is some redundancy in identifying a track. That is, a track will have an identifiable servo pattern as well as an assigned track identifier. This redundancy provides a more robust track identification scheme to resolve the least significant bit (LSB) of the gray code. This permits head position to be determined even if there is noise in the demodulated PES signal or other difficulty in position determination.
FIG. 2 is a representation of a conventional disk drive quad-burst servo pattern in which magnetic transitions are recorded on the disk surface in bursts labeled as A, B, C, and D. The servo pattern bursts move relative to a disk drive magnetic head (not illustrated) from right to left. The disk data tracks and half-track positions are indicated by the track numbers along the left side of the FIG. 2 drawing. The portion of the disk 22 shown in FIG. 2 extends approximately from track N-1.0 toward the inner diameter of the disk half-track position N+2.5 toward the outer diameter. Those skilled in the art will appreciate that position information is decoded by demodulating the signal generated by the head passing over the PES burst patterns to form a primary signal P based on: EQU P=A-C
and to form a quadrature signal Q based on: EQU Q=B-D.
The signals P and Q are quadrature signals because they are cyclic and are out of phase by 90 degrees (one-quarter phase). The magnetic transitions that comprise the servo pattern are represented in FIG. 2 by vertical bars. The letter within each group of bars represents the PES burst recorded therein. One burst is distinguished from another by relative position in a track and relative position to the other bursts. Thus, for a read head that can detect magnetic transitions from more than one track at a time, the signal P should be zero when tracking exactly along the centerline of track N, because the head will detect equal amounts of magnetic field from the A and C servo bursts. A similar situation exists for tracks N+1, N+2, and so forth for tracks that are an even number multiple of half tracks from N. For the half-track position N+0.5, the signal Q should be zero when tracking exactly along the N+0.5 half-track "centerline", because the head will detect equal amounts of field from the B and D servo bursts. The signal Q should be zero also for half-track positions N+1.5, N+2.5, and so forth.
To fit increasing amounts of customer data on disk storage systems, customer data transfer rates are increasing. This can necessitate increased servo data rates, because the servo channel and data channel often share filters. Increased servo data rates make it more difficult to align gray codes from track to track, which increases the incidence of misreading the gray code. These gray code misread errors can cause disk seek errors and errors in reading and writing data, if severe enough. A further advantage of increasing the servo data rate is that the gray code then requires less disk surface area than before, so more of the disk track area can be used for customer data.
From the discussion above, it should be apparent that there is a need for a disk drive system with a track identification scheme that can identify many thousands of tracks on a disk without requiring a gray code. The present invention fulfills this need.